The present invention relates to hydraulic pumps or motors of the radial cylinder type.
With reference to FIG. 1 of the annexed drawings, a conventional radial cylinder hydraulic pump or motor comprises a stator with a hollow guide element 1 having a cylindrical inner surface 2 and a rotor 3 which is rotatably mounted within the guide element 1 around an axis 4 which is offset from the axis of the cylindrical surface 2. The rotor 3 has a number of radial cylinders 5 wherein radial pistons 6 are slidably mounted. Each piston 5 is provided at its radially outer end with a guide pad 7 which slides on the inner surface 2 of the guide element 1. A fixed cylindrical body 8 is arranged within an inner cavity of the rotor 3 and is provided with inlet and outlet ducts for the oil. In the operation as a pump, the rotor 3 is caused to rotate by a motor (not shown) so that the radial pistons 6 are urged outwardly by centrifugal force. In some cases, springs or fluid pressure coming from an auxiliary pump are exploited in order to increase the thrust urging the radial pistons outwardly.
In the above described conventional construction, the pads 7 are pivotally connected to the respective pistons and are pressed against the surface 2 of guide element 1 with the interposition of a film of oil under pressure fed through a hole formed through the pad itself. Such a construction is not completely satisfactory, since it poses a limitation to the offset degree of axis 4 and to the speed of the rotor.
From U.S. Pat. No. 2.130.299 to Ernst a radial piston pump or motor is known wherein a continuous series of rollers is provided covering the inner surface of the guide element 1 and interposed between each piston pad and the inner surface of the guide element. In this case, however, each pad is connected to the respective piston through the engagement of the outer end of the piston in a seat of the pad. Also, this known construction can be operated only with a minimun offset degree of the rotor axis and therefor poses again a limitation to the capacity and power of the pump. The reason of this limitation is that, due to the offset position of the rotor axis, each pad and the outer end of the respective piston have a displacement relative to each other also in a tangential direction with respect to the rotor axis. Moreover, power losses occur as a consequence of the contact between adjacent rollers, which have opposed local speeds in the point of contact.